Cryolitic vat for the production of aluminum by electrolysis

ABSTRACT

DISCLOSED ARE MONOCELL AND MULTICELL FURNACES FOR FUSED BATH ELECTROLYSIS OF ALUMINA (AL2O3) DISSOLVED IN MOLTEN FLUORIDE BATHS. THE INVENTION IS CHARACTERIZED BY AN INTERNAL VAT CONSTITUTED ENTIRELY OR PARTIALLY OF SUBSTANTIALLY PURE CRYOLITE, EITHER NATURAL OR SYNTHETIC.

Oct. 24, 1972 e.- as 'VARDA 1 CRYOLITIC V AT FUR. THE PRODUCTION OF ALUMINUM,BY ELECTROLYSIS v Original Filed July e, 19e'r United States Patent M US. Cl. 204-243 R 7 Claims ABSTRACT OF THE DISCLOSURE Disclosed are monocell and multicell furnaces for fused bath electrolysis of alumina (A1 0 dissolved in molten fluoride baths. The invention is characterized by an internal vat constituted entirely or partially of substantially pure cryolite, either natural or synthetic.

The present application is a continuation of application Ser. No. 651,448, filed July 6, 1967.

As is known, conventional monocell furnaces, of either the Soederberg type having self-baking anodes or of the type having prebaked anodes, for the electrolytic production of aluminum, use vats lined with carbonaceous material. These vats contain the bath of molten fluorides, e.g. molten cryolite, in which alumina (A1 0 is in solution. In operation of these furnaces, the alumina dissolved in the fuse] bath is electrolyzed with metallic aluminum forming and collecting on the bottom of the carbon vat, which acts as a cathode.

'It is known that said carbon vat containing the fused bath and the molten metal produced, is insulated with respect to the outside by other layers of refractory and thermally insulating material. The successive layers of carbonaceous material and of refractory and insulating materials are encompassed in an external metal shell.

The electric current, which is D.C., leaves that cathodic vat through metal bars, usually of iron, which connect the internal bottom layer of the vat, consisting of carbonaceous material, which is a good current conductor, to external collectors of current-carrying bus bar systems which in turn connect electrically in series the long row of electrolysis furnaces in a furnace room (pot-room) with conventional electrolysis furnaces for the production of aluminum.

It is known that this type of cathodic vat possesses many inconveniences of which two in particular are:

1) The rapid spoiling of the carbon side walls (ledges) due to the chemical and electrochemical aggressiveness of the fluorinated bath and to variations in composition and temperature of the bath during the operation of the furnace, e.g. anodic effect, A1 0 feed, tapping of Al, adjusting the inter-electrodic distance, etc. The variations in temperature and composition of the bath lead to continuous solidifications and redissolutions of the bath which seeps through the ledges, and causes, in the course of a few weeks, spoiling of the carbonaceous material of those side walls. Danger that the fluid bath, once it has passed through the carbonaceous layer forming the inner edge of the carbon vat, would escape, i.e. leak from the metal shell which usually is at a temperature between 50 C. and 200 C., however does not exist in practice. Cryolite baths have a notorious tendency to solidify as soon as the temperature of the electrolysis bath drops by 50 or 100 C. below the electrolysis temperature which is about 950 C.

A semisolid to solid layer of solidified bath forms in the inner part of the carbon vat and replaces in part or en- 3,700,581 Patented Oct. 24, 1972 tirely the pre-existing carbonaceous layer, at least as far as the carbon side-walls are concerned, possibly because these are not protected by a layer of molten aluminum.

As known, these self-formed linings of solidified or semisolid cryolitic material are of variable thickness and never or almost never attain the optimum thickness for a really rational operation of the furnace. Temperature variations of the bath in running the furnace, even if these variations are not extended in time and are only of the order of magnitude of a few tens of degrees, appreciably influence the thickness of the solidified bath, i.e. regressing of the semisolid side walls. In fact, a variation of the bath temperature by a few tens of degrees, easily causes variations in the thickness of the side walls consisting of solidified cryolite material, up to some centimeters. The inner walls of conventional monocel furnaces thus transform gradually and spontaneously in such a way as to be constituted less and less of carbonaceous material but more and more of cryolite material of rather variable composition having, however, a relatively low melting temperature, i.e., lower than 940 C. In other words, said sidewalls are scarely solid and are semisolid when in contact with the bath; the side-walls have variable and on the average little suitable thickness and are unstable and unreliable.

(2) On the other hand, other phenomena and inconveniences are observed at the bottom of the carbon vat. First of all, a perimetrical ring, consisting of a sort of mass from solid to highly viscous, frequently forms on the bottom of the carbon vat. This mass in actual practice,

- especially if it is beneath the layer of cathodic aluminum,

is almost impossible to re-dissolve in the molten bath.

Swellings and deformations of the carbon bottom also occur by the bath components oozing through and by local overhea-tings due to a lessening uniform distribution of the electrical current in the so deformed and impregnated carbon bottom.

The consequence is that the ohmic drop of the electrical current passing through said bottom before leaving through the metal bars increases. This results in a marked increases, usually from 1 to 3 kw. h./kg. of Al produced, of the per unit energy consumption in respect to that of modern electrolysis furnaces provided with new vats. The latter figure is about 15 kw. h./kg. of Al produced.

Hence, those conventional vats must be periodically disconnected, demolished and built with considerable waste of time, losses of production and materials, and

labor costs.

Multicell furnaces run at reduced amperages but with over-all voltages much higher than those of the conventional monocell furnaces. In these furnaces, an internal vat (side walls and bottom) cannot be made of carbonaceous material in direct contact with the molten bath, since such a vat would give rise to serious phenomena of current by-pass and parasitic electrolysis between the various electrodes suspended in the molten bath and the aforementioned carbon vat.

Many materials have been suggested for lining or replacing those carbonaceous materials, both in the sidewalls of conventional monocell furnaces and the side walls and the bottom of multicell furnaces.

It had been believed till now that these protective and/ or substitute materials should contemporaneously have a group of characteristics not likely to be found in practice in one and the same material, i.e.:

(l) to resist easily temperatures well above 1000 C. and

to have excellent refractory characteristics;

(2) to resist Well chemical action and electrochemical attack of the components of fluorinated electrolysis baths;

(3) to resist well the attack and penetration of liquid aluminum;

(4) to exhibit, at least in multicell furnaces, a high ohmic resistance at electrolysis temperature, even if impregnated with liquid bath.

The material which so far had most approached those characteristics, so that a certain number of conventional mouocell furnaces have walls (but only side-walls) lined with such material, is silicon nitride-bonded silicon carbide. This material, as presently available to the trade, has however the defect, besides being very expensive, of also having, when hot and immersed in the cryolite bath, a relatively low ohmic resistivity. Although it resists well chemical attack (in particular in the cathodic zone), it poorly resists, in multicell furnaces, the electrochemical attack of the current that passes through the fluorinated baths to flow along a portion of the vat wall.

The drawing shows a typical multicell aluminum furnace. The particular furnace is of the necklace type.

The structural details of this furnace can be found in US. Pat. No. 3,178,363. The furnace hereof corresponds to that shown in FIG. 6 of said Pat. No. 3,178,363, the pertinent portions of which are incorporate by reference.

The vat 1 containing the bath is made of carbonaceous material and is lined on its entire inner surface by refractory layer 2. The vat 1 is protected on the outside by an insulating jacket 3, providing thermal insulation. The bipolar electrodes 4 are rigidly suspended from supporting bars 7 fastened to longitudinal beams 17. The bars are fastened to the beams 17 by collars 19. Each bar 7 is electrically insulated from its suspension beam by an insulator 20. The beams 17 are also electrically insulated from the remainder of the furnace by insulators (not shown). The current-supply connecting bars 28 of the necklace furnace serve also to suspend monopolar electrodes 4'. The consumable anode portion of each electrode is fed from the top through a stack, not shown in the drawing. Both the bipolar electrodes 4 and the terminal monopolar electrodes 4' are framed with a protective refractory coating that is inert both to the bath and to the electrolysis. The refractory frame comprises the side coatings 6, the base coatings 22 and the top coatings 43.

The central longitudinal refractory wall 12 is provided with vertical pockets 13 for reception of the metal produced. The metal produced in any one of the cells is conveyed to the corresponding pocket 13 through individual grooves 25, suitably dimensioned and arranged on the vat bottom to take account of the bath circulation and preferably having an inclined bottom. The pockets 13 are connected through a conduit 29 with the groove 25 of the inclined bottom. An overflow weir 33 serves to let the molten aluminum overflow into a receptacle 31 common to each series of cells.

Some materials made of cryolite-alumina mixtures and containing at least 20% by weight, preferably more than 40% by weight of alumina, have been proposed recently (see US. Pats. 3,093,570; 3,261,699; 3,267,183 and French Pat. 1,353,565) for lining the bottom and/or the walls of traditional furnaces for alumina reduction. These materials, which are not current conductors, can replace entirely the carbon linings, thus overcoming their above-mentioned shortcomings. Further, these materials allow the use of current collectors immersed in molten aluminum deposit and which are made of borides, nitrides and carbides of elements found in groups 4, 5 and 6 of the periodic table (e.g. TiB These current collectors, however, cannot be used, because of their brittleness, in conjunction with the carbon linings which, since they swell and heave during furnace operations, would cause shearing and cracking of said collectors.

It is pointed out that the above-mentioned prior art provides the use of cryolite always in mixture with alumina.

Furthermore, the compositions preferred by said prior art contain at least 40% by weight of alumina and particularly as far as the construction of side-walls is cona 4 cerned, the above-mentioned French patent points out as the most suitable mixtures those containing to by weight of alumina. Therefore, alumina is an essential component of said mixtures. This can be explained by considering the following data:

(a) The low melting point of cryolite (-1000 C.) as

compared to that of alumina (2050 0.), both in rela- I have surprisingly found that, in contrast with the prior art referred to hereinbefore, when applying cryolitic materials in the construction of internal refractory vats of multicell furnaces, it is not only unnecessary to use alumina as a mixture component with cryolite, but it is technically advantageous to dispense with the alumina.

I have found, and this forms part of the invention as hereinafter described, that a material which is constituted of natural or synthetic cryolite as pure as possible can be employed for the protection of the internal side-walls of monocell furnaces and especially for the internal fining (side-walls and bottom) of multicell furnace vats. The cryolite material can partially or completely replace the same carbon walls. The cryolite has been melted for instance in a small furnace for the electrolytic production of aluminum, preferably fed with alternating current, and subsequently poured into molds or forms (of carbon, silicon nitride-bonded silicon carbide such as sold under the trade name Refrax" by Carborundum, metal or other suitable material) having desired shapes.

As known, the melting point of substantially pure cryolite natural or synthetic is between 970 and 1000 C. This is therefore above the normal operating temperatures of conventional or multicell furnaces for fused bath electrolysis for aluminum production, which as stated above are usually lower than 950 C.

The vat or the shaped pieces (for instance bricks and plates) or the vat parts thus cast, can 'be easily welded with one another by merely heating the vat once the individual pieces have been assembled. Joints are thus obviated (always being weak points in any other constructions) by simple adhesion of the individual pieces to the adjacent ones when hot. If such treatment should not immediately provide a perfect weld, one need do nothing else but wait for the subsequent cautious start of the furnace, for the final elimination of those joints.

Said vats, once assembled in situ, can be easily completed towards the outside, with the usual refractory and heat-insulating material, obtaining the welding with the external layers by a layer of rammed carbonaceous paste. In many cases, however, these refractory and heat-insulating materials are superfluous and the external metal vat only (pot shell) sufiices.

The hcreinbefore described prefabricated cryolite material has a high melting point and is not subject to chemical or electrochemical attacks by the bath. There remains, however, the possibility that the 'bath while at contact with said material, may progressively dissolve it and this more rapidly the more the diffusion will be facilitated of the so dissolved materials towards the middle of the bath. In order to ensure the stability as to size and chemical composition, the duration and the safety (prevention of the bath from leaking out) of the side-walls of the internal vat built with said materials, it is advisable that the temperature of the electrolysis bath in the layer at contact with the prefabricated vat should be kept as low as possible.

This can be attained for example by dimensioning the multicell furnace so that the electrodes which are suspended in the bath should have a distance of 20 to 30 cm. and preferably 30 to 40 cm. and more, from the longitudinal side-walls of the cryolite vat. In such case, if the furnace is designed and operated correctly, the temperature of the bath/ prefabricated material contact layer will be lower than 925 C., preferably lower than 900 C. Furthermore, the heat insulation of side-walls and the thickness of the cryolitic layer, which may be reduced even down to a few centimeters, have to be suitably dimensioned.

In multicell furnaces, the part of the internal vat that causes the greatest difiiculties when the electrolysis furnace is working, is the vat bottom. The bottom, no longer an electrical conductor, as contrasted to the bottoms of conventional vats in which a remarkable localized heat development takes place by the Joule effect, tends to cool down and to cause the freezing of the overlying bath. In this case, descent of the produced aluminum towards the bottom is restrained, or even prevented, by these layers of half-frozen bath. The aluminum cannot reach the special collecting pits, thereby prejudicing in practice the satisfactory performance of the multicell furnace.

To obviate such inconvenience, it is advisable in multicell furnaces that the bottom of the vat containing the bath should be as close as possible to the lower border of the bipolar electrodes suspended in said bath.

In this case, however, the vat bottom of prefabricated material, when in contact with too hot bath layers, would be subjected to a certain wear due to dissolution in the bath. This inconvenience can be easily avoided by protecting the prefabricated cryolite vat bottom against too hot a bath either by special refractory materials different from the cryolite materials having high melting point (for instance by refractory materials based on silicon nitridebonded silicon carbide or by a layer molten Al, or by both (for instance pits for collecting the molten aluminum, provided in the bottom of the prefabricated cryolite vat and lined with such special refractory material).

In short, said prefabricated cryolite vats having solid walls, practically are not subject in practice to any chemical or electrochemical attack and, if adopting the suitable contrivances already described, not even to the dissolving power of the electrolysis bath. The cryolitic material of said vats can be considered a real antifluorine refractory material. Said vats are very poor conductors of electric current. The cost of these vats is very low, since the starting material costs less (especially if compared to the cost of silicon nitride-bonded silicon carbide) and their preparation is very simple.

The cryolitic material which the furnace vats according to the present invention are made of, shows the further great advantage in comparison with the cryolite-alumina mixtures of allowing the construction of big shaped vertical walls.

As a result of correct furnace operation these cryolitic vats are very stable and do not disintegrate even after protracted runs.

It should be noticed that the invention has a particular importance in the construction of multicell furnaces, where, because of the use of bipolar electrodes, the sidewalls are much higher than in traditional furnaces.

Further, the refractory linings made of cryolite-alumina mixtures, show in comparison with the cryolitic material according to the present invention, the drawback of needing mixing operations for their preparation. Moreover, mixtures with a high alumina content need to be heated up to temperatures which are much higher than the melting point of substantially pure cryolite (for instance, 1350-1450" C. for mixtures containing 60-85% by wt. of alumina) with great energy waste and operating difficulties.

The invention has been disclosed in an embodiment with the use of prefabricated shaped cryolitic pieces. Of course, the scope of the invention is not restricted to such embodiment but also includes its variants and equivalents.

For instance, a further method for constructing a furnace vat according to the present invention is to introduce into the metal shell, which may be lined with a heatinsulating material and which constitutes the outer part of the furnace pot, a shaped body (e.g. of carbon) so as to leave a clear space between said body and the inner walls and/or bottom of said lined shell. Then molten cryolite is poured into said clear space to fill it. The cryolitic material is allowed to solidify, whereafter the said body is removed. The term cryolite as used in the claims refers to natural or synthetic cryolite.

I claim:

1. A monocell or multicell furnace for the electrolysis of alumina dissolved in molten fluoride baths, comprising an external shell and an internal refractory lining vat, said vat only being composed of substantially pure cryolite.

2. The apparatus of claim 1, wherein the internal refractory vat includes at least one prefabricated shaped piece of substantially pure cryolite.

3. The apparatus of claim 2, containing at least one protective material above the entire vat bottom along all of the vat sides for preventing said bottom and sides from coming in contact with bath layers which act as solvents.

4. The apparatus of claim 3, wherein the protective materials consists of solid refractory materials based on silicon nitride-bonded silicon carbide.

5. The apparatus of claim 2, wherein the vat bottom is of pure cryolite.

6. The apparatus of claim 2, wherein both the vat bottom and vat sides are of pure cryolite.

7. In a vat of a furnace for the electrolytic production of primary aluminum from alumina dissolved in a fluorinated fused bath, a solid refractory layer consisting at least for a portion of the surface of said lining, only of cryolitic material of external origin, substantially pure, prefabricated and shaped for the inner lining of the vat, which lining when in operation will be in direct contact with the fused bath of electrolyte, also molten aluminum.

References Cited UNITED STATES PATENTS 3,261,699 7/ 1966 Henry 204-243 R X 3,267,183 8/1966 Feinleib 204-243'R X 3,457,158 7/1969 Bullough 204243 R 3,475,314 10/1969 Johnston 204-243 R JOHN H. MA'CK, Primary Examiner D. R. VALENTINE, Assistant Examiner US. Cl. XJR. 204244 

